1. Field of the Invention
The present invention relates to dimeric diol-modified polyaddition compounds containing uretdione groups, a process for their production and their use as cross-linking agents for thermally cross-linkable powder coating compositions.
2. Description of the Prior Art
Combinations of blocked polyisocyanates and organic polyhydroxyl compounds, which are solid at room temperature, are valuable binders for thermally cross-linkable powder coating compositions (see e.g. DE-OS 2,105,777, DE-OS 2, 542 191, DE-OS 3,143,060, DE-OS 2,735,497, DE-OS 2,842,641, EP-A 286,799, EP-A 218,040, EP-A 460,963, DE-OS 2,801,126, EP-A 403,779, WO 91/15532, U.S Pat. No. 3,857,818, U.S. Pat. No. 4,375,539, EP-A 409,745, DE-OS 2,812,252, DE-OS 2,946,085 and DE-OS 3,434,881).
A common disadvantage of the systems in these prior publications is that during thermal cross-linking, the compounds used as blocking agents split off and escape into the environment. Consequently, for environmental and health reasons, special precautions have to be taken when working with these materials in order to purify the waste air and/or recover the blocking agent.
One attempted means of overcoming this disadvantage is to use polyisocyanate cross-linkers that contain uretdione groups, but are free from blocking agents. The products are cross-linked by thermal re-cracking of the uretdione groups.
Manufacture of these powder cross-linking agents is known. For example, DE-OS 2,420,475 describes cross-linking agents for polyurethane (PUR) baking systems. These cross-linking agents are free from blocking agents and are obtained by reacting polyisocyanates or polyisocyanate mixtures containing uretdione groups with aliphatic or cycloaliphatic compounds containing one or two, preferably two isocyanate-reactive groups. In particular, solid cross-linking agents having melting points of 140.degree. to 260.degree. C. are obtained by reacting simple diols, optionally containing ether groups, with polyisocyanates containing uretdione groups and prepared from aromatic diisocyanates, such as toluylene diisocyanate (TDI).
Since splitting of the uretdione group in the presence of hydroxy-functional reactants occurs at temperatures as low as about 110.degree. C., the cross-linking agents cannot be manufactured in the melt and must be prepared in solvents inert towards isocyanates, which necessitates an additional step for removing the auxiliary solvent.
Powder cross-linking agents containing uretdione groups and prepared from straight-chain difunctional IPDI uretdione, simple diols optionally containing ether groups and optionally chain terminating monoamines or monoalcohols are disclosed in EP-A 0,045,994, EP-A 0,045,996 and EP-A 0,045,998. In the manufacture of these polyaddition products, the reaction temperature must not exceed 110.degree. C. in order to prevent splitting of the uretdione groups. More particularly, substances melting at or even above the splitting temperature are produced when there is a high degree of chain extension, which is required for cross-linking agents having a high uretdione functionality and consequently a high cross-linking density, i.e., when at least 70% of the NCO groups in the difunctional IPDI uretdiones are reacted with diols to provide chain extension. Consequently these polyaddition compounds cannot be reliably and reproducibly manufactured except in solution, which results in a subsequent evaporation step.
The IPDI uretdione powder cross-linking agents described in the cited prior publications have melting points of about 80.degree. C., considerably below the splitting temperature, but they have such high melt viscosities at the maximum permitted reaction temperature of 110.degree. C. that the possibilities of producing them in the melt on an industrial scale are limited, because it is not possible to provide adequate agitation.
Also, due to their high melt viscosity, powder lacquers produced form the known cross-linking agents containing uretdione groups have mediocre optical properties, particularly inadequate levelling properties.
An object of the present invention is to provide PUR powder cross-linking agents containing uretdione groups that do not suffer from the previously described disadvantages of prior art polyaddition products.
This object has been achieved with the polyaddition compounds according to the invention described in detail hereinafter. These polyaddition compounds are based on the surprising observation that polyaddition compounds containing uretdione groups and manufactured with the additional use of diol components in the form of dimeric alcohols, have such low melt viscosities, even at temperatures only slightly above their melting point, that they can be produced without difficulty in the melt and can be used to obtain powder coatings having excellent levelling properties and very high gloss. A particularly surprising fact is that PUR powder coating compositions, which contain the polyaddition compounds according to the inventions as cross-linking agents, harden to form completely cross-linked coatings at appreciably lower baking temperatures than the corresponding polyaddition compounds in the prior art, which are manufactured without using dimeric alcohols.